Significant effort has been invested over the years in improving the energy and power of electrical energy storage devices such as capacitors and batteries. These are particularly useful in short term, high energy applications such as electric vehicles or cellular communication. Warren et al., U.S. Pat. No. 5,418,682, discloses a capacitor having an electrolyte consisting of tetraalkylammonium salts which include nitrile or dinitrile solvents and teaches that such a system provides a high dielectric constant, which is needed for capacitance. Morimoto et al., U.S. Pat. No. 4,725,926, discloses a non-aqueous electrolyte comprising quaternary phosphonium salts in organic solvents for use in electric double layer capacitors.
A wide variety of solvents and salts are available for such use, offering specific advantages depending on the application being considered (e.g., low temperature vs. high temperature). Ionic liquids based on the imidazolium cation have recently received attention as non-aqueous electrolytes in various electrochemical devices (Koch et al., J. Electrochem. Soc. 143:155, 1996). These electrolytes have significant advantages compared to the numerous quaternary onium salts that have been previously investigated for use in carbon double-layer capacitor electrolytes (Ue et-al., J. Electrochem. Soc. 141:2989, 1994).
Electrochemical capacitors based on nonaqueous electrolytes offer greater electrochemical stability (up to 4 V) compared to aqueous systems, thereby providing greater energy storage (E=½CV2). However, due to the lower conductivity of nonaqueous electrolytes compared to aqueous systems, lower power capabilities are observed. In addition, with the porous materials used in electrochemical capacitors, the high viscosity typically associated with the high dielectric constant solvents used in non-aqueous electrolytes is detrimental to conductivity in porous electrodes.
There is an ongoing need for stable more conductive solutions of ionic salts for electronic devices such as batteries and double layer capacitors. In the case of lithium ion batteries, it is the lithium salts that are required such as lithium hexafluorophosphate is a suitable stable solvent such as an organic carbonate mix. In the case of the double layer capacitor it is usually an organic solvent solution of a quaternary ammonium salt such as tetrafluoroborate in acetonitrile or organic carbonates. These battery and double layer capacitor salts are usually used in the ranges of about 0.6 to 2.0 molar depending on their solubility and where the maximum conductivity falls. Usually in the case of double layer capacitor electrolytes, the solutions have conductivities of up to about 55 mS-cm at about 1 molar in acetonitrile using tetraethylanmonium tetrafluoroborate.